[slang] Fix virtual interface comparison

[likeamahoney]

systemverilog LRM (for example IEEE 1800-2017 or IEEE 1800-2023) 25.9 clause says:

— Equality ( == ) and inequality ( != ) with the following: • Another virtual interface of the same type • An interface instance of the same type • The special constant null

It follows that virtual interfaces can be compared with instances of the same type which is not supported by slang for now.

I fixed this at that PR.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.75%. Comparing base (cad562f) to head (4fc070b).

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[MikePopoloski]

Thanks for the PR!